The invention relates generally to optical aberration measurement and analysis, and more particularly to an objective measurement of optical systems, such as systems of a human eye.
Optical systems having a real image focus can receive collimated light and focus it at a point. Such optical systems can be found in nature, e.g., human and animal eyes, or can be man-made, e.g., laboratory systems, guidance systems, and the like. In either case, aberrations in the optical system can affect the system""s performance. By way of example, the human eye will be used to explain this problem.
A perfect or ideal eye diffusely reflects an impinging light beam from its retina through optics of the eye which includes a lens and a cornea. For such an ideal eye in a relaxed state, i.e., not accommodating to provide near-field focus, reflected light exits the eye as a sequence of plane waves. However, an eye typically has aberrations that cause deformation or distortion of reflected light waves exiting the eye. An aberrated eye diffusely reflects an impinging light beam from its retina through its lens and cornea as a sequence of distorted wavefronts.
There are a number of technologies that attempt to provide the patient with improved visual acuity. Typically, treatment is determined by placing spherical and/or cylindrical lenses of known refractive power at the spectacle plane (approximately 1.0-1.5 centimeters anterior to cornea) and literally asking the patient which lens or lens combination provides the clearest vision. This is an imprecise measurement of true distortions in the reflected wavefront because 1) a single spherocylindrical compensation is applied across the entire wavefront, 2) vision is tested at discrete intervals (i.e., diopter units) of refractive values, and 3) subjective determination by the patient is employed. Thus, conventional methodology for determining refractive errors in the eye is substantially less accurate than the techniques now available for measuring the ocular aberrations.
One method of measuring ocular refractive errors is disclosed in U.S. Pat. No. 5,258,791 to Penney et al. for xe2x80x9cSpatially Resolved Objective Autorefractometer,xe2x80x9d which teaches the use of an autorefractometer to measure the refraction of the eye at numerous discrete locations across the corneal surface. The autorefractometer is designed to deliver a narrow beam of optical radiation to the surface of the eye, and to determine where that beam strikes the retina using a retinal imaging system. Both the angle of the beam""s propagation direction with respect to the optical axis of the system and the approximate location at which the beam strikes the corneal surface of the eye are independently adjustable. However, a small uncertainty or error in the location of the beam""s point of incidence on the cornea exists due to the curved corneal surface. For each point of incidence across the corneal surface, the refraction of the eye corresponding to that surface point can be determined by adjusting the angle at which the beam strikes the cornea until the beam refracted on to the iris strikes the fovea centralis. Adjustment of the beam angle of propagation can be accomplished either manually by the patient or automatically by the autorefractometer, if a feedback loop involving a retinal imaging component is incorporated.
Penney ""791 further teaches the use of the autorefractometer measurements in determining the appropriate corneal surface reshaping to provide emmetropia, a condition of a normal eye when parallel beams or rays of light are focused exactly on the retina and vision is perfect. This is accomplished by first obtaining an accurate measurement of corneal surface topography using a separate commercially available device. A mathematical analysis is then performed using an initial corneal topography at each surface reference point, the measured refraction at each surface point, and Snell""s law of refraction, to determine a desired change in surface contour at each reference point.
A major limitation to the approach described by Penney ""791 is that a separate measurement of corneal topography is desired to perform the Snell""s Law analysis of needed refraction change. This adds significantly to the time and cost of a complete and desirable diagnostic evaluation. Further, the accuracy of the refraction change analysis will be dependent upon the accuracy of the topographic measurement and the accuracy of the autorefractometer measurement. In addition, any error in the spatial orientation of a topography map with respect to a refraction map will degrade the accuracy of the measured profile. Yet another limitation to known approaches such as described in Penney ""791, by way of example, is that test points on the corneal surface are examined sequentially. Eye motion during the examination, either voluntary or involuntary, could introduce substantial errors in the refraction measurement. Penney ""791 teaches detection of such eye movement by deliberately including measurement points outside the pupil, i.e., in the corneal region overlying the iris, where the return from the retina will obviously be zero at specific intervals in the examination sequence. However, this approach may still allow substantial undetected eye movement error between such iris reference points.
By way of example, one method and system known in the art, are disclosed by Junzhong Liang et al. in xe2x80x9cObjective Measurement Of Wave Aberrations Of The Human Eye With The Use Of A Hartmann-Shack Wave-Front Sensor,xe2x80x9d published in the Journal of the Optical Society of America, Volume 11, No. 7, July 1994, pages 1949-1957. Liang et al. teach the use of a Hartmann-Shack wavefront sensor to measure ocular aberrations by measuring the wavefront emerging from the eye by the retinal reflection of a focused laser light spot on the retina""s fovea. The actual wavefront is reconstructed using wavefront-estimation with Zernike polynomials.
The imprecise measurement technique of placing lenses of known refractive power anterior to the cornea and asking a patient which lens or lens combination provides the clearest vision has been improved with the use of autorefractometers, as described in Penny ""79, or with the use of wavefront sensors as described by Liang et al. Spatially resolved refraction data, in combination with measured existing surface contour of the anterior surface of the eye, enable a calculation of a detailed spatially resolved new contour. However, it would be an improvement in this art if such vision measurements could be made without the need for this contour data, and further without the need for feedback from the patient regarding an appropriate lens. Liang et al. discloses the use of a Hartmann-Shack wavefront sensor to measure ocular aberrations by measuring the wavefront emerging from the eye by retinal reflection of a focused laser light spot on the retina""s fovea. A parallel beam of laser light passes through beam splitters and a lens pair which brings the beam to a focus point on the retina by the optics of the eye. Possible myopia or hyperopia of the tested eye is determined by movement of a lens within the lens pair. The focused light on the fovea is then assumed to be diffusely reflected and acts as a point source located on the retina. The reflected light passes through the eye and forms a distorted wavefront in front of the eye that results from the ocular aberrations. The aberrated wavefront is then directed to the wavefront sensor.
A point source of radiation on the retina would be ideal for such measurements. However, when the perfect eye receives a collimated beam of light, the best possible image on the retina is a diffraction limited spot. As illustrated by way of example, with Penny et al. and Liang et al., discussed above, and typical for those of skill in the art, parallel or collimated beams are used with the optics of the eye being measured to achieve this diffraction limited spot for such objective measurements. To do so, a setup for each patient includes a lens or lens combination and adjustments thereto for accommodating that patient""s specific visual acuity. Providing a lens combination, as well as setting up for their use becomes cumbersome, time consuming, and at an additional expense. Eliminating the need for such optics is desirable and eliminates a variable within optical measurement systems that typically include many variables. Further, there is a need for providing optical characteristics of an eye without requiring feedback from the patient. By way of example, the patient may be a wild or domestic animal, living or dead.
The Hartmann-Shack wavefront sensor disclosed by Liang et al. includes two identical layers of cylindrical lenses with the layers arranged so that lenses in each layer are perpendicular to one another, as further disclosed in U.S. Pat. No. 5,062,702 to Bille. In this way, the two layers operate as a two-dimensional array of spherical lenslets that divide the incoming light wave into sub-apertures. The light through each sub-aperture is brought to focus in the focal plane of the lens array where a charge coupled device (CCD) image module resides.
The system of Liang et al. is calibrated by impinging an ideal plane wave of light on the lenslet array so that a reference or calibrating pattern of focus spots is imaged on the CCD. Since the ideal wavefront is planar, each spot related to the ideal wavefront is located on the optical axis of the corresponding lenslet. When a distorted wavefront passes through the lenslet array, the image spots on the CCD are shifted with respect to a reference pattern generated by the ideal wavefront. Each shift is proportional a local slope, i.e., partial derivatives of the distorted wavefront, which partial derivatives are used to reconstruct the distorted wavefront, by means of modal wavefront estimation using Zernike polynomials.
However, the system disclosed by Liang et al. is effective only for eyes having fairly good vision. Eyes that exhibit considerable myopia (nearsightedness) would cause the focus spots to overlap on the CCD, thereby making local slope determination practically impossible for eyes having this condition. Similarly, eyes that exhibit considerable hyperopia (farsightedness) deflect the focus spots such that they do not impinge on the CCD thereby again making local slope determination practically impossible for eyes having this condition.
In general, an embodiment of the present invention provides a method and system for objectively measuring aberrations of optical systems by wavefront analysis. Another embodiment further provides for the objective measurement of ocular aberrations having a dynamic range that can cope with large amounts of such aberrations so as to be useful in practical applications. Still another embodiment of the present invention provides a method and system for objectively measuring ocular aberrations using a wavefront analyzer of simple and inexpensive design.
One embodiment of the present invention provides an apparatus and method for making objective and detailed measurements of aberrations present in human eyes.
Aberrations measured by the apparatus include xe2x80x9chigher orderxe2x80x9d phenomena, such as spherical aberration and coma, in addition to the traditional myopia/hyperopia and astigmatism.
In accordance with an embodiment of the present invention, an energy source generates a beam of radiation. Optics, disposed in the path of the beam, direct the beam through a focusing optical system that has a rear portion which provides a diffuse reflector. The beam is diffusely reflected back from the rear portion as a wavefront of radiation that passes through the focusing optical system to impinge on the optics. The optics project the wavefront to a wavefront analyzer in direct correspondence with the wavefront as it emerges from the focusing optical system. A wavefront analyzer is disposed in the path of the wavefront projected from the optics and calculates distortions of the wavefront as an estimate of ocular aberrations of the focusing optical system. The wavefront analyzer includes a wavefront sensor coupled to a processor that analyzes the sensor data to reconstruct the wavefront to include the distortions thereof.
One embodiment of the present invention, herein described by way of example, utilizes wavefront sensing to measure the aberrations of the eye. When one considers the perfect or ideal eye as earlier described, a perfectly collimated light beam (i.e., a bundle of parallel light rays) incident on the perfect, ideal emmetropic eye, focuses to a diffraction-limited small spot on the retina. This perfect focusing is true for all light rays passing through the entrance pupil, regardless of position. From the wavefront perspective, the collimated light represents a series of perfect plane waves striking the eye. Due to the reversible nature of light ray propagation, the light emanates from an illuminated spot created on the retina as wavefronts exiting the ideal eye as a series of perfect plane waves. The apparatus of the present invention achieves this ray reversal effect using a probe beam optical path for projecting a small diameter, eye-safe laser beam into the eye and onto the fovea. The light scattered from the irradiated retina serves as a secondary source for a re-emitted wavefront. The probe laser beam strikes the retina at an appropriate foveal location to illuminate a sufficiently small spot. A fixation optical path is provided which includes a reference target aligned to an optical axis. This allows a patient to fixate on a target. A video path provides a video image of the eye plane, centered on the optical axis. A video image of the eye allows a clinical operator to assist in orienting the eye for the wavefront measurement.
Embodiments of the present invention, herein described, provide a refraction measurement system that easily accommodates the measurement of vision characteristics of the eye, even in the presence of finite refractive errors. The time for a patient to be in a fixed position during examination is reduced, while at the same time providing a useful source of light on the retina of the eye to be measured regardless of the characteristics of the eye of that patient or other patients to be examined. Desirably, measurements are made without requiring patient or operator feedback. One method aspect of the invention for measuring optical characteristics of an optical system, such as the eye, includes focusing an optical beam onto an anterior surface of the eye for providing a finite source of secondary radiation on the retina of the eye, which secondary radiation is emitted from the retina as a reflected wavefront of radiation that passes through the eye. The reflected wavefront is directed onto a wavefront analyzer for measuring distortions associated with the reflected wavefront.
In one embodiment, the radiation is optical radiation and the wavefront sensor is implemented using a plate and a planar array of light-sensitive cells. The plate is generally opaque but that has an array of light transmissive apertures that selectively let impinging light therethrough. The plate is disposed in the path of the wavefront so that portions of the wavefront pass through the light transmissive apertures. The planar array of cells is arranged parallel to and spaced apart from the plate by a selected distance. Each portion of the wavefront passing through one of the light transmissive apertures illuminates a geometric shape covering a unique plurality of cells.
As herein described, by way of example, the wavefront optical path of the present invention relays the re-emitted wavefront from the corneal plane to an entrance face of a Hartman-Shack wavefront sensor. The wavefront incident on the sensor is received by a sensitive charged-coupled device (CCD) camera and an optical plate containing an array of lenslets. The lenslet array is parallel to the CCD detector face with a distance therebetween approximately equal to the focal length of each lens in the lenslet array. The lenslet array divides the incoming wavefront into a matching array of xe2x80x9cwavelets,xe2x80x9d each of which focuses to a small spot on the CCD detector plane. The constellation of wavelet spots in the CCD is used to reconstruct the shape of the incident wavefront. Collimated light striking the lenslet at normal (perpendicular) incidence would focus to the spot on the CCD face where this optical axis intersects. The optics of the apparatus provides such collimated light to the wavefront sensor using a calibration optical path. Collimated light CCD images are routinely obtained as part of a daily calibration process and used for reference in analyzing experimental data.
However, in the case of a reflected aberrated wave front, light focuses to a spot displaced from the collimated reference point by a distance Dx. The distance from the lenslet face to the CCD surface, Dz, is precisely known. Therefore, dividing the measured displacement, Dx, by the known propagation distance, Dz, the slope of the wavefront at the location of this lens element is determined. The same calculation is applied in the y direction within the plane, and the entire process applied to every lenslet element irradiated by the wavefront. A mathematical algorithm is then applied to reconstruct the wavefront shape consistent with the calculated Dx/Dz and Dy/Dz slope data. Regardless of which wavefront sensor is used, the distance between the planar array of cells and the opaque plate, or the array of lenslets, can be varied to adjust the slope measurement gain of the wavefront sensor and thereby improve the dynamic range of the system.
Another measure of dynamic range enhancement is provided by the focusing optics. The focusing optics includes first and second lenses maintained in fixed positions in the path of the beam and wavefront. An arrangement of optical elements is disposed between the lenses in the path of the beam and the wavefront. The optical elements are adjustable to change the optical path length between the lenses.
A method aspect of the present invention, as herein described, determines aberrations of an eye requiring greater than a + or xe2x88x923 diopter change, and includes directing an optical beam onto a retina of an eye, reflecting the optical beam from the retina of the eye, determining characteristics of a wavefront in a reflected optical beam, and generating data based on the characteristics of the wavefront, which data quantifies the aberrations of the eye. The data may further be generated based on refractive indices of media through which the optical beam passes. Yet further, data based on the characteristics of the wavefront, which data quantifies the aberrations of the eye for a discrete section of the eye may also be generated.
One method for determining aberrations of an eye, herein described by way of example, includes directing a probe beam along a probe beam path toward an eye, directing a fixation image along a fixation image path toward the eye, directing a light source along a video image path toward the eye, generating a video image of the eye, directing a wavefront originating from the eye along a wavefront path, wherein the probe beam path, the fixation image path, the video image path, and the wavefront path are coincident at least along a portion of their respective paths, the probe beam path terminating at the retina of the eye and the probe beam reflecting from the retina of the eye as a wavefront, aligning the eye with the probe beam path based at least in part on the video image of the eye generated by the light source directed along the video image path, measuring the wavefront, and generating data representative of the aberrations of the eye based on the measurement of the wavefront. Further, the aligning of the eye with the probe beam path based at least in part on the video image of the eye generated by the light source directed along the video image path, may have the wavefront pass through a single microlens array.
One apparatus for determining the aberrations of an eye comprises a patient head rest comprising vertical adjustment, the patient head rest associated with an optical table having a base. The base carries a probe beam generating apparatus, probe beam directing optics, the probe beam directing optics comprising a beam splitter; a mirror; and a lens, the probe beam directing optics being capable of directing a probe beam toward an eye of a patient positioned on the patient head rest, video image components, the video image components comprising a light source, a mirror, and a video camera, the video image components being capable of generating an image of an eye of a patient positioned on the patient head rest, eye fixation components, the eye fixation component comprising a fixation target; a light source; a lens; and a mirror, the fixation components being capable of generating a target that the eye of a patient positioned on the patient head rest can view, and wavefront directing and analyzing components, the wavefront directing and analyzing components comprising a lens, a mirror, a microlens array, a camera, and a data processor. The wavefront directing and analyzing components are capable of measuring the wavefront emanating from the eye of a patient positioned on the patient head rest and determining aberrations of said eye that range from at least about + or xe2x88x921 diopters to at least about + or xe2x88x926 diopters.